Transforming multiple objects to have equal dimensions
Just press the Alt key before clicking any of the value text boxes and inserting you values.

That will affect all selected objects instead of just the active one. It will also work for modifiers and most other properties.



Edit

If all your planes belong to the same object you have to separate them before controlling sizes directly, since absolute size control will only work in Object Mode, not Edit Mode.


Enter edit mode in your planes object
Select all with A
Separate all planes with P > Loose Parts
Now exit edit mode
Select all separated plane objects
Move their object center to the center of the geometry so their positions wont change when you change their scale, by pressing Shift + Ctrl + Alt + C > Origin to Geometry
Now you can change size


Don't forget to apply transforms afterwards with Ctrl + A > Scale